It's probably fair to say that on the whole, the volatile keyword in Java is poorly documented, poorly understood, and rarely used. To make matters worse, its formal definition actually changed as of Java 5. On this and the following pages, we will cut through this mess and look at what the Java volatile keyword does and when it is used. We will also compare it to other mechanisms available in Java which perform similar functions but under subtly different other circumstances.

What is the Java volatile keyword?

Essentially, volatile is used to indicate that a variable's value will be modified by different threads.

Declaring a volatile Java variable means:

The value of this variable will House Brown Warm Pattern Slippers Fleece JadeRich Unisex Cute Bear never be cached thread-locally: all reads and writes will go straight to "main memory";

Access to the variable acts as though it is enclosed in a synchronized block, synchronized on itself.

We say "acts as though" in the second point, because to the programmer at least (and probably in most JVM implementations) there is no actual lock object involved. Here is how synchronized and Unisex Bear Brown House Fleece Slippers JadeRich Warm Pattern Cute volatile compare:

In other words, the main differences between synchronized and volatile are:

a primitive variable may be declared volatile (whereas you can't synchronize on a primitive with synchronized);

an access to a volatile variable never has the potential to block: we're only ever doing a simple read or write, so unlike a synchronized block we will never hold on to any lock;

because accessing a volatile variable never holds a lock, it is not suitable for cases where we want to read-update-write as an atomic operation (unless we're prepared to "miss an update");

a volatile variable that is an object reference may be null (because you're effectively synchronizing on the reference, not the actual object).

Attempting to synchronize on a null object will throw a NullPointerException.

Volatile variables in Java 5

We mentioned that in Java 5, the meaning of volatile has been tightened up. We'll come back to this issue in a moment. First, we'll look at a typical example of using volatile. Later, we'll look at topics such as: